The specific objectives of this project include 1) to determine the analog and dose of retinoid that can prevent/delay mammary tumor development, 2) to determine the retinoid and tamoxifen combination effect on delaying mammary tumors development and 3) to determine the retinoid receptor pathway involved in delaying/preventing the development of mammary tumors. For this purpose female transgenic (TG.NK) mice with human breast cancer oncogene "neu" that develop 50% incidence of mammary tumors by ~30 weeks of age will be treated with selected doses of retinoids to establish the most effective retinoid and dose. The most effective retinoid analog will be further evaluated in combination with tamoxifen to determine the synergism between the two drugs.